Attempts have been made to minimise errors caused by deficiencies in the Cathode Ray Tube (CRT) or Photo-Multiplier Tube (PMT). Shading correction has been used on TV cameras and telecine for many years and has generally taken the form of a set of waveforms which are predefined or adjusted to be the reciprocal of the shading error, these waveforms being multiplied by (in the case of linear signals), or added to (for logarithmic signals) the uncorrected video signals to produce signals which are substantially free from shading errors.
This method of correction is satisfactory for smooth and symmetrical errors such as may be caused by non-uniform lens transmission characteristics. However, errors caused by variations in efficiency of the phosphor layer in CRTs are not consistent and require the combination of adjustable proportions of many different waveforms to achieve a satisfactory correction. The same considerations are true for variations in cathode sensitivity of PMTs and graduations of colour filters. Consequently, to obtain satisfactory correction, a large number of controls requiring a complex alignment is necessary. Even then it is not possible to correct isolated patches on the picture.
Another type of error is known as burn. Burn errors are those caused by the scan spending a longer time at some locations on the CRT face than at others. The result is localised solarisation of the glass or staining of the phosphor which results in distinct steps in the light output at those locations. Burn errors cannot be corrected by the type of shading corrector described above since they generally display a series of sharp edges. It has been proposed to use a separate burn corrector which uses an additional PMT which looks directly at the CRT face to measure the burn errors, and then calculates the reciprocal of the error waveform, before multiplying it together with the video waveforms to produce corrected video waveforms.
This proposed method has not proved wholly satisfactory as it has the disadvantage that non-uniformities in the burn correction optical path or due to burn PMT sensitivity variations (which may be very significant) will be applied to the video signal, resulting in a more difficult task for the shading corrector.
A further source of errors are blemishes and dirt. Blemishes are small sharply focused spots of no light output from the CRT which are caused by missing particles of phosphor or debris on the phosphor surface. Dirt on the CRT faceplate will also appear as dark spots.
Neither of these errors can be corrected by the shading corrector outlined above. However, one or other of the dirt or blemish errors can be corrected using the burn corrector described. Both errors cannot be corrected at once as the burn PMT is off the optical axis and thus gives parallax errors so that when adjusted to correct for blemishes it produces a correction for dirt at the wrong picture location. Fitting the burn PMT on the optical axis is undesirable as a mirror system would have to be used involving a consequent loss of light and deterioration in the telecine signal to noise ratio.
Another source of errors is the phosphor grains themselves. The granular structure of the CRT phosphor results in random variations in the light output which are of small size and amplitude. These errors cannot be corrected by the shading corrector described above but can be improved by the burn corrector when adjusted to minimise blemishes.
Various methods have been proposed for compensating for variations in response in telecine and television cameras. BBC Research report BBC RD 1985/3 discloses a system which compensates for variations in sensitivity in individual elements of a line array CCD sensor. This is known as a "stripe stripper" and operates by measuring the response of each element of the 1024 element line array. In effect, the system multiplies the output of each element by a correction factor derived for that element to give a substantially uniform output across the array and to eliminate the vertical stripes which result from variations in response across the array.
GB 2149260 (Marconi) discloses another CCD compensation system. The principle is similar to the proposal of BBC RD 1985/3 but applied to area array CCDs. Thus, a correction factor is derived for each element of array.
GB 2074416 (Ampex) relates to television cameras and divides an active video picture into a 13 block 14 band ratio. In a setup mode the signal from a selected camera is sent to an A/D converter in the video signal path. Horizontal and vertical error measurements are made in which selected samples within blocks of successive horizontal lines and bands of vertical lines are summed to provide horizontal and digital data which are then subtracted from the measured output of each block or band to derive a correction factor.
None of the above proposals considers how to compensate for the type of defects which arise in flying spot telecine. Furthermore, the two documents which compensate for variations in sensitivity of individual CCD elements do not consider how to compensate for other defects which arise in the optical path, for example uneven film illumination.